Wagner B. De Almeida

Ab initio calculations of electric field gradient for the B…HCl dimers: B=CO, HCN, H2O, H2S, C2H2, C2H4, NH3, PH3 and CH3CN

Abstract Ab initio calculations of electric field gradient have been performed for the H-bonded directs B…HCl, where B=CO, HCN, H 2 O, H 2 S, C 2 H 2 , C 2 H 4 , NH 3 , PH 3 and CH 3 CN, with an STO/4-31G basis set. The influence of geometry relaxation on the properties is investigated. The nuclear quadrupole coupling constant χ( 35 Cl) has been evaluated for each dimer and its change from the free HCI subunit value (χ 0 ) upon direct formation analyzed. Total atomic charges, dipole and quadrupole moments were also calculated. Comparison with a recent experimental work is given.

An ab initio study of the CO2…C2H2 binary complex

Abstract The carbon dioxide-acetylene binary complex has been investigated at the ab initio SCF/4-31 G, SCF/D95 and SCF/6-31G** level of theory. Two stationary points have been located on the potential energy surface (PES): the “parallel” and “linear” structures. Harmonic vibrational frequencies, IR and Raman intensities have been calculated analytically. Rotational constants and the intermolecular harmonic force field are also given. The SCF/4-31G results are shown to be in very good agreement with recent experimental gas-phase near-infrared spectroscopic studies.

Ab initio investigation of the stationary points on the potential energy surface for the CO2…HCN binary complex

Abstract An ab initio SCF study of the potential energy surface for the complex between carbon dioxide and hydrogen cyanide has been carried out with three different basis sets: STO/4–31G, D95 and STO/6–31G**. Two energy minima have been located at linear and T-shaped structures. Harmonic frequencies and vibrational intensities were calculated analytically and are reported along with other spectroscopic parameters. MP2/6–31G** stabilization energies were also calculated and they predict the linear complex to be marginally more stable than the T-shaped structure observed experimentally.

Ab initio vibrational spectrum of HCN monomer, dimer and trimer

Abstract Vibrational frequencies and intensities calculated at the SCF-MO level with an STO/4-31G basis set are presented for the HCN monomer, linear dimer and linear trimer. Both fully optimised geometries are given along with frozen monomer geometries, the primary aim being to investigate the effect of geometry relaxation on the infra-red parameters.

Ab initio vibrational spectrum of the H-bonded trimers (HCN)2HF and HCN(HF)2

Ab initio SCF calculations of harmonic vibrational frequencies and I.R. and Raman intensities of the normal modes are reported for the H-bonded trimers: HCN … (HF)2, (HF)2 … HCN, HF … HCN … HF, HCN … HF … HCN, HCN … HCN … HF and HF … HCN … HCN. Calculations with basis set of different levels, STO/4-31 G and D95, are presented. Rotational constants and one-electron properties are given and the assignments of the I.R. spectra discussed.

An AM1 study of conformational preference for the herbicide atrazine

Abstract The potential energy surface for atrazine, a commercially important herbicide, has been extensively investigated using the quantum mechanical semiempirical molecular orbital method AM1. Full symmetry unconstrained geometry optimisation was performed for each stationary point located on the energy surface. The relative stabilities and rotational barriers for the conformera were also calculated. Four minimum energy conformera were predicted to occur with respect to rotation about the triazine ring carbon atoms and the amino ethyl and amino isopropyl groups. A relatively low energy barrier to bond rotation of 30 kJ mol−1 was predicted. The conformation adopted by triazine molecules on protein binding suggests that this conformation is principally stabilised by intermolecular hydrogen bonding with the protein active site.

The infrared spectrum of the methylidyne-phosphine dimer, (HCP)2

Abstract In this paper, we report ab initio geometries, infrared frequencies and intensities for the HCP dimer. Two levels of theory have been used, restricted Hartree—Fock (RHF) and Moller—Plesset second-order perturbation theory (MP2). Significant reduction of the intermolecular distance is found between the RHF and MP2 optimizations. At the MP2 level, only one dimer, the “T” shaped, is found to be a minimum. The infrared frequencies and intensities exhibit the typical characteristics of a weakly interacting system, which along with the MP2 zero-point dissociation energy of −3.5 kJ mol−1, classes this dimer as a van der Waals complex.

An ab initio study of the C2N2 molecule: NCCN, CNNC and CNCN species

The linear species NCCN, CNNC and CNCN have been studied at the ab initio SCF level with the basis set STO/4–31G, STO/4–31G∗∗ and D95. Electron correlation was taken into account at the MP2/D95 and CISD/D95 level. Harmonic vibrational frequencies and infrared and Raman intensities were evaluated analytically as well as the harmonic force field. Dipole moments, quadrupole moments, electric-field gradients and nuclear quadrupole coupling constants were calculated. Rotational constants and vibrational transition moments are given.

A computational thermodynamic evaluation of the altitude profiles of (N2)2, N2-O2 and (O2)2 in the Earth's atmosphere

Abstract The equilibrium constants of formation of the gas phase species (N 2 ) 2 , N 2 -O 2 and (O 2 ) 2 are used for evaluation of the altitude population profiles in the Earths atmosphere. The equilibrium constants are derived from recently computed quantum-chemical data. Estimates of the lower bounds of the interaction energies are employed so that the reported data should also represent a lower bound of the populations. The mole fractions of each of the dimers, computed for their equilibrium mixture with N 2 and O 2 under atmospheric conditions, decrease with altitude. The mole fractions decrease in the order (N 2 ) 2 , N 2 -O 2 , and (O 2 ) 2 . The (N 2 ) 2 mole fraction close to the surface is found to be 8 × 10 −4 %, i.e. comparable with other minor components of the Earths atmosphere currently being vigorously studied.

Ab initio molecular structure and vibrational spectra of the acetylene trimer

Abstract The potential-energy surface (PES) for the acetylene trimer, (C 2 H 2 ) 3 , has been investigated and two stationary points located. The global minimum is predicted to have a cyclic configuration. Vibrational frequencies and intensities have been calculated at the ab initio SCF level with an STO/4-31G basis set. The relative stability of the two trimers is discussed and comparison with recent experimental results given.